Session A18: Focus Session: Carbon Nanotubes: Synthesis and Growth I

Super Growth Carbon Nanotubes

Water assisted CVD (denoted as Super Growth) results in a significant enhanced catalyst activity and enlonged lifetime of the catalysts to synthesize carbon nanotubes. The high efficient growth results in massive growth of vertically-aligned single-walled nanotubes forests with heights up to 2.5 millimeters and carbon purity over 99.98{\%}. Super Growth simultaneously addresses many critical problems such as scalability, purity, and cost, and opens up innumerable opportunities ranging from fundamental research to real applications. This presentation will provide an overview of our recent development of the ``Super Growth'' CVD. First, the synthesis of highly efficient impurity free SNWT forest will be described. Second, the growth dynamics will be explored with our recent advance in CNT synthesis, as well as characterizing the physical and chemical properties of SWNT forests. Third, various new forms of carbon nanotube material such as DWNT forests, SWNT solids made by utilizing the super-growth technique will be demonstrated with emphasis on their applications such as super-capacitors. Lastly, challenges and future projects that are planed will be summarized.   

Density functional study of cyclacene-based carbon nanotubular compounds.

We use {\em ab initio} Density Functional calculations to investigate the interplay between structural and electronic properties of a new class of one-dimensional nanowires, related to carbon nanotubes. The cyclacene building blocks consist of phenyl rings, and can be viewed as the shortest segments of (n,0) zigzag nanotubes. In our study, we focus on cyclacenes with n=6-12 phenyl rings, and compare our results to infinitely large cyclacenes, corresponding to narrow graphene ribbons. The nanowires are formed by inter-connecting cyclacenes to a chain using biphenyl, tetrazine, or acetylene linkers. Depending on the nature and the orientation of the linkers, we find it possible to change the systems from narrow- to wide-gap semiconductors, and to modulate the band dispersion, suggesting the possibility of band gap engineering. We will also discuss the relevance of our results for a diameter- and chirality-selective synthesis of carbon nanotubes.   

The mechanism for low temperature growth of vertically aligned boron nitride nanotubes

Boron nitride nanotubes (BNNTs) are well recognized as the candidate that will complement the uses of carbon nanotubes (CNTs) in nanotechnology. However, high growth temperatures ($>$1100 $^{o}$C), low production yield, and impurities have prevented effective synthesis and applications of boron nitride nanotubes (BNNTs) in the past ten years. For the first time, we have succeeded on the growth of pure BNNTs on substrates [1, 2]. This has been realized based on our experiences of growing CNTs and boron nitride (BN) phases (cubic phase BN, hexagonal phase BN). According to our hypothetical model, energetic growth species play an important role on controlling the phases of BN solids. We have experimentally verified that BNNTs can be grown by energetic growth species by a plasma-enhanced pulsed laser deposition (PEPLD) technique. These BNNTs can be grown vertically aligned into arrays of regular patterns at 600 $^{o}$C, and can be used for applications without purification. The growth mechanism of thee BNNTs will be discussed. [1]. Yap et al., Bull APS Vol 50, 1346-1347 (March 2005). [2]. Wang \textit{et al., nano Letters }(2005) ASAP, DOI: 10.1021/nl051859n.   

Selective Growth of Single Wall Carbon Nanotubes from Superparamagnetic Maghemite

The growth of carbon nanotubes in specific configurations and geometry is crucial to developing useful applications in nanoelectronics and smart coatings. We have shown that single wall carbon nanotubes (SWNT) of diameters less than 2 nanometers can be grown directly from catalyst particles comprising of maghemite ($\gamma -$Fe$_{2}$O$_{3})$ on a Silicon substrate using the conventional chemical vapor deposition process. The sizes of SWNT were measured using Atomic Force Microscopy. The average tube diameter was measured to be 1.5$\pm $0.2 nm. Scanning Electron Microscopy measurements revealed that the catalyst oxide particles formed in clusters of 100 nm diameters. Transmission M\"{o}ssbauer measurements at room temperature showed the presence of a magnetic sextet corresponding to maghemite with a particle size $>$ 100 nm and a superparamagnetic phase with particle size less than 20 nm. Our results also indicate that the superparamagnetic phase of maghemite with average particle size of about 5 nm plays a critical role in the formation of SWNT with specific tube dimensions. Experiments are currently underway to characterize the relaxation rates of the superparamagnetic phase of maghemite. The fundamental role of arrays of metal nano-catalysts and superparamagnetic nanoclusters of maghemite in the selective growth of single wall carbon nanotubes will be presented.   

Controlling the height of CVD-grown multi-wall nanotube arrays

Due to various difficulties, carbon nanotube arrays have seen only limited use in industrial applications to date. One of the difficulties is the reproducible growth of these arrays, let alone a good measure of control over the obtained height. In this work, we have preformed a parametric study of multi-wall carbon nanotube (MWNT) growth. The investigated parameters were gas flow rate, process pressure, and water content of the feed gas. We were able to identify a region in the parameter space that yields stable and highly reproducible growth of tall nanotubes arrays. As a result, we can controllably grow MWNT forests to any height between 1 $\mu $m and 1 mm by choosing the right combination of pressure, humidity, flow rate, and growth time. Additionally, we were able to perform kinetic studies of the carbon nanotube growth, and our results suggest that the precursor for nanotube growth is formed in the gas phase.   

Real-Time Study of the Kinetics of Vertically Aligned Single Wall Carbon Nanotube Array Nucleation and Growth

A molecular beam of carbon containing molecules in conjunction with time resolved reflectivity was used to study the kinetics of nucleation and growth of vertically aligned single wall carbon nanotube arrays. The molecular beam environment decouples the source gas and the substrate temperature dependent variables and eliminates secondary gas phase reactions, to allow carbon nanotube growth by surface reactions only [1]. The incidence rate of the carbon containing species is the key variable that through the nucleation density determines all the important properties of the arrays including the type, the diameter, and the packing density of the nanotubes. The addition of trace amounts of impurities such as water and oxygen enhances the nucleation density but does not affect growth. This highly controlled reaction environment reveals that carbon nanotube growth is a complex multicomponent reaction in which not just C but also H and O play a critical role. The picture that emerges form this study is at odds with the conventionally accepted dissolution/precipitation model for carbon nanotube growth. Instead, we explain the observed results by a new mechanism that is based on carbon network formation and stabilization by stepwise addition of acetylene type species. [1] G. Eres \textit{et al}. J. Phys. Chem. B \textbf{109}, 16684 (2005).   

Defect induced modification in thermal property of Regioreguler Poly(3-hexylthiophene) nanotube composites

The interaction particularly, interfacial bonding between polymer and filler has remained a crucial phenomenon to be understood to optimize their uses in many practical applications. Up to now, most of the work is on the chemical fuctionalization for improving nanotube/matrix interaction. In this paper, we studied the effects of ion irradiation induced defects on thermal behaviors of Poly(3-hexylthiophene) nanotube composites using Thermogravimetry Analysis (TGA) and Differential Scanning Calorimetry (DSC). Where, the irradiation is used to introduce the defects in a control way on pristine nanotube before composite formation. Several interesting effects were observed; including thermal stability enhancement and defects induced enhanced interaction between nanotube and polymer, and substantial changes in spectroscopic behaviors of the composites due to irradiation.   

Effect of graphitic order on the electron field emission of carbon nanotube films

Carbon Nanotubes (CNTs) are known to be excellent electron field emitters. However, the fundamental factors that contribute to the emission stability have not been well studied. Here, we found that stability of emission current from CNTs is related to their graphitic orders. We have tested various types of CNTs grown by thermal chemical vapor deposition (CVD) and plasma enhanced CVD (PECVD). Our samples were grown in a circular area of 0.385cm$^{2}$ on low resistance Si substrates. Field emission measurements were conducted in a planar diode configuration, with a pair of electrodes separated with a gap of 1000 $\pm $ 10 $\mu $m. The vacuum level during the measurement is $\sim $2.0 X 10$^{-7}$ mbar. We found that the emission currents from PECVD grown CNTs degraded by as much as 70{\%} within a period of 20 hours. In contrast, random CNTs grown by thermal CVD exhibit stable emission current for at least 20 hours. These CNTs also have relatively lower threshold electric field of field emission. Since all samples are tested in a same condition, the detected results are thus related to the structural order of the CNTs. Transmission electron microscopy and Raman spectroscopy confirmed that field emission stability is depends on the graphitic structures of these CNTs.   

Structure and Applications of Nanoporous Carbon

Recently fractal networks of nanopores in activated carbon have been discovered (Pfeifer et al., Phys. Rev. Lett. 88, 115502 (2002)). We study the formation and properties of these networks with the goal of using them to store methane at low pressures (Alliance for Collaborative Research in Alternative Fuel Technology, http://all-craft.missouri.edu). Van der Waals forces in the nanopores force methane into a dense fluid (supercritical adsorption). We investigate the pore structure by nitrogen and methane adsorption isotherms, small-angle x-ray scattering (SAXS), and electron microscopy, and use the data to model the formation of the pore network using probabilistic cellular automata on a lattice. The calculated scattering from our simulated networks is in close agreement with experimental SAXS data. The models are designed to give us a deeper understanding of the growth of these networks and allow us to optimize their properties. Currently our best sample stores 0.11 g methane per cm$^{3}$ monolithic carbon at 25 $^{o}$C and 34 atm (90{\%} of industry target).   

Boron-doped SWNTs for Electromagnetic Interference Shielding

SWNTs have been proposed for electromagnetic interference shielding as an additive in polymers. However, 2/3 of the SWNTs are expected to be semiconducting and only 1/3 are metallic. B-doping of graphite is known to lead to strongly p-doped material. In SWNTs, doping is therefore expected to lead to degenerately-doped semiconducting tubes and then a shielding benefit can be derived from all tubes in the sample. We find that 1-2 at{\%} B-doping is possible at 25-50 gr/hr production. The quality of the HCl-purified product has been investigated by Raman scattering (RS), optical adsorption, transmission electron microscopy and temperature programmed oxidation . RS spectra were found to exhibit sharp G,R bands and a very weak D-band component is also observed. We note that after B-doping the 2$^{nd}$ order RS cross section is enhanced, as reported previously for pulsed laser produced B-doped SWNTs by Rao et al. All band positions are very close to that observed for undoped SWNTs, indicating a reasonably good molecular structure of the B-SWNTs and consistent with the small size of atomic boron (i.e., substitutional dopant). OA show that the interband absorption peaks associated with semiconducting and metallic tubes upshift by 20-50 meV, indicating p-doping.   

Detachment and re-attachment of vertically aligned single-walled carbon nanotube films

A hot-water assisted detachment method of CVD-grown vertically aligned single-walled carbon nanotube (VA-SWNT) films from substrates has been developed. In particular, we found that the VA-SWNT films is efficiently peeled off by submersing the substrate into heated ($\ge $ 60 \r{ }C) distilled water, and the detached film floats on the water surface. Furthermore, the detached film is readily re-attached to arbitrary surfaces. SEM observation confirms that the aligned morphology is perfectly preserved even after the re-attachment to other substrates. Mechanism of the proposed hot-water assisted film detachment method is investigated and suggested.   

Fabrication and characterization of nanopatterned ultrathin epitaxial graphite films.

High quality ultrathin graphite films, composed of less than 10 graphene layers, have been epitaxially grown on single crystal SiC substrate by thermal decomposition. Hall bar structures, top gate and side gated field transistor structures have been fabricated using electron beam lithography methods. Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM) and Electrostatic Force Microscopy (EFM) have been used to characterize the structures. These results demonstrate that nanopattened epitaxial graphite (NPEG) is a promising quasi-two-dimensional electronic material. .   

Structural modification of boron nitride nanotubes by plasma irradiation

Boron nitride (BN) and boron-carbon-nitride (B-C-N) nanotubes (NTs) are candidates for potential nanosized electronic and optical devices due to extraordinary physical and chemical properties. In terms of electronic property, in contrast to the insulating BNNTs with about 5.5 eV band gap, ternary B-C-N NTs has semiconducting property, the band gap of which is primarily determined by their chemical compositions. Although one of the methods to make B-C-N NTs is C doping to BNNTs, it is difficult to modify the structure and composition of BNNTs due to its chemical inertness and strong sp2 bond. In this study, we performed to modify the structure and composition of BNNTs by plasma irradiation for synthesizing B-C-N NTs. Hydrocarbon plasma was utilized for structural modification of BNNTs. The structural properties and the composition were characterized by high-resolution transmission electron microscopy and electron energy loss spectroscopy. After the plasma irradiation, outer several BN layers were modified to wavy structure from straight shape, and the defects were observed in almost BN layers, indicating destruction of crystal structure by collision of energetic particles in plasma and BNNTs. There are 5 -- 30 at{\%} of C in BNNTs and C atoms were inhomogeneously distributed in B-C-N NTs. The electron transport property of the modified B-C-N NTs will be reported in our presentation.